Size press programs for print quality improvement, Solutions!, Online Exclusives, June 2005, Vol. 88(6)

For uncoated printing and writing papers, the implementation of an appropriate size press program can address on-press performance issues of filler dusting and fiber linting. While starch, as well as an appropriate foam control agent, are typically found in a size press solution, new high performance programs can make the difference in delivering a sheet that performs as expected.

During the offset printing process, the paper surface is subjected to both mechanical and chemical treatments. Mechanically, the paper surface is forced against the printing blanket. During this ink transfer step, slippage between the blanket and paper occurs, resulting in abrasion to the paper surface. Any abraded material removed from the sheet will remain with the printing blanket. As the ink is transferred, it “splits” between the paper surface and the printing blanket. This separation exerts shear forces such that the paper surface may be disrupted. Any components of the paper not firmly attached at the surface will be removed and, again, deposit on the blanket.

While subjected to these mechanical forces, both an aqueous fountain solution and oil containing ink are being applied to the sheet’s surface. With this treatment, components of the sheet may dissolve into the fountain solution. Some may remain with the printing blanket in the non-image areas containing the fountain solution. The organic solvents present in the ink will also interact with the components of the paper. Along with the shear forces and chemical involvement, the length of time in which the printing blanket, ink, and aqueous fountain solution are in contact with the paper surface will also have an impact on the print job. The nature of an acceptable offset sheet, then, must be such that it will readily accept the mechanical and chemical treatments typical of offset printing.

With increased use of short fiber for greater surface smoothness as well as mineral fillers for increased optical properties and reduced fiber content, an on-press, blanket contamination problem referred to as “piling” arises with greater frequency. Piling is the accumulation of pigments and fillers from paper, ink, or of fibers or dust from the paper.

Dusting
Pigment dusting is one form of piling. Dusting results from the removal of loosely bonded pigments or fillers from an uncoated paper. These undesirable dust particles will be present on the blanket in both image and non-image areas. Dusting inhibits the development of the desired tone density. Should the dust be calcium carbonate, a reaction with acidic fountain solution may occur with resultant blanket “whitening or milking.” Correction requires that technicians stop the press and perform a wash-up.

Linting
Fiber linting is another type of piling. It results when loose fiber on the surface of the paper accumulates on the printing blanket. Such material will also interfere with the uniform transfer of ink.

Linting can make for poor uniformity in solid tones. As with dusting, excessive linting requires the pressman to stop the print job and perform a wash-up.

Ink piling
Should both dust and lint accumulate on the printing blanket and remain undetected, a severe piling problem can occur. With time, a resultant build-up in the image areas takes on the color of the ink and appears as a putty-like mass. This mass, formed on the second and succeeding stations of an offset press, can become increasingly tacky, which hinders ink transfer and flow. Color development will deteriorate. Correction requires a halt to production with subsequent removal of the piled material.

While the causes for piling problems such as pigment dusting and fiber linting are numerous, Nalco’s experience in pigment coating and surface sizing applications has led to the development of size press programs that focus on print quality improvement. These programs include surface modifiers and sizing agents. Such programs are tailored to fit the needs and performance requirements of a specific grade.

Click to see larger image.
KRK water interference mottle test before (left)
and after the Nalco program.Nalco has developed surface modifiers to alter the sheet surface properties so that the interaction between the paper and printing blanket will not impart any undesirable abrasion. Furthermore, with surface modifier treatment, filler pigments will remain with the sheet and not “dust.” When a sizing agent is also used, the treated paper surface responds better to the mechanical shear forces of the split of the offset inks. On-press linting can be minimized or even eliminated. The short fibers present on the sheet for smoothness remain with the sheet. A mill using this approach has significantly altered the on-press performance of its highly filled, opaque grades.

About the author:
John Krasniewski is senior technical consultant, Nalco Company, Naperville, Illinois, U.S. To learn more about Nalco’s size press programs, visit www.nalco.com or call 1-630-305-2641.